The rapid advancement of semiconductor fabrication technology results in ever increasing component density and integration density of semiconductor devices. Currently, transistors are widely used as the basic semiconductor devices. As the component density and integration density increase, the gate dimension of transistors is getting smaller. However the shrinking gate dimension of transistors may result in short channel effect, which often causes leakage current and ultimately affects electrical characteristics of resultant semiconductor devices.
In order to improve transistor performance, to suppress short channel effect and to reduce leakage current, one method includes use of a semiconductor-on-insulator (SOI) substrate to form transistors. The semiconductor-on-insulator substrate includes a substrate, an insulating layer on the surface of the substrate, and a semiconductor layer on the surface of the insulating layer. The material of the substrate may be mono-crystalline silicon, the material of the insulating layer may be silicon oxide and the material of the semiconductor layer may be silicon or germanium.
The transistor gate structure is formed on the surface of the semiconductor layer. The transistor drain region and source region are formed on the surface of the semiconductor layer on both sides of the gate structure. The channel region is formed in the semiconductor layer at the bottom of the gate structure. Since the bottom of the semiconductor layer is insulated by the insulating layer, it is difficult for the carriers in the channel region of the semiconductor layer to enter the substrate under the insulating layer. The short channel effect may be suppressed and the leakage current in transistors may be reduced.
As the size of semiconductor devices continues to shrink and the integration density continues to increase, the size of transistors formed shrinks correspondingly. The fabrication method adapted to the reduced transistor size may use a semiconductor-on-insulator substrate with ultra-thin body and buried oxide (UTBB). The ultra-thin body indicates the semiconductor layer and the buried oxide indicates the insulating layer on the substrate. The semiconductor-on-insulator substrate with ultra-thin body and buried oxide (UTBB) is a substrate with a thin semiconductor layer and a thin insulating layer. The thin thickness of the semiconductor layer and the insulating layer reduces the transistor size formed, suppresses the short channel effect and reduces the leakage current in transistors.
However the transistors formed by employing the conventional semiconductor-on-insulator substrate with ultra-thin body and buried oxide (UTBB) still provide unstable performance.